Adds the widgets block editor to widgets.php and customize.php

Moves the widgets block editor from Gutenberg into WordPress Core.

- Adds @wordpress/edit-widgets, @wordpress/customize-widgets and
  @wordpress/widgets.
- Modifies wp-admin/widgets.php to branch between the old editor and new editor
  depending on wp_use_widgets_block_editor().
- Modifies WP_Customize_Widgets to branch between the old editor control and new
  editor control depending on wp_use_widgets_block_editor().

Fixes #51506.
Props isabel_brison, TimothyBlynJacobs, andraganescu, kevin940726, talldanwp.


git-svn-id: https://develop.svn.wordpress.org/trunk@50996 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/edit-form-blocks.php

@@ -29,6 +29,14 @@ $block_editor_context = new WP_Block_Editor_Context( array( 'post' => $post ) );
 $current_screen = get_current_screen();
 $current_screen->is_block_editor( true );
 
+// Default to is-fullscreen-mode to avoid jumps in the UI.
+add_filter(
+	'admin_body_class',
+	function( $classes ) {
+		return "$classes is-fullscreen-mode";
+	}
+);
+
 /*
  * Emoji replacement is disabled for now, until it plays nicely with React.
  */